An Electron Diffraction Study of the Effect of Heat upon the Structure of Gold Leaf

The construction of an electron diffraction apparatus is described, adapted to both reflection and transmission studies of crystalline materials and built in the laboratory at a moderate cost. This apparatus involves several novel features of construction and utilizes an electrode magnet for obtaining a homogeneous beam of electrons. The application of the instrument to chemical problems is illustrated by the results of a study of the effect of heat and of various gaseous atmospheres upon the structure of gold leaf. Microscopic, x-ray and electron diffraction evidence indicate that at a temperature of about 400° the gold in a leaf flows into aggregates or larger crystal grains. This coagulation explains the contraction of the leaf as observed visually, the opacity of many patches in the heated leaf as observed microscopically, and the appearance of spots in the diffraction patterns. A ``many ring'' pattern observed when the leaf is heated in air or oxygen is attributed to the formation of an oxide which is rather unstable at about 400°. This observation clarifies some discrepant data in the literature.